Polarizing plates having a structure in which a protective film is laminated on one or both surfaces of a polarizer formed with a polyvinyl alcohol (hereinafter, referred to ‘PVA’)-based resin dyed with dichroic dye or iodine using an adhesive have been commonly used. Triacetyl cellulose (TAC)-based films have been normally used as a polarizing plate protective film in the art, however, such TAC films have a problem of being readily deformed in high temperature and high humidity environments. Accordingly, protective films made of various materials capable of replacing TAC films have been recently developed, and for example, a method of using polyethylene terephthalate (PET), a cycloolefin polymer (COP), and an acryl-based film either alone or as a mixture thereof has been proposed.
Herein, aqueous adhesives formed with an aqueous solution of a polyvinyl alcohol-based resin are normally used as an adhesive used to attach the polarizer and the protective film. However, aqueous adhesives have a problem in that the use is limited depending on the material of a film, since adhesive strength is weak when acryl-based films or COP films and the like are used as the protective film instead of TAC. In addition to the problem of adhesive strength defects depending on the materials, the aqueous adhesive also has problems in that curls are generated in a polarizing plate due to a drying process of the aqueous adhesive, and initial optical properties are degraded when materials of the protective film used on both surfaces of a PVA element are different. Moreover, a drying process is absolutely required when the aqueous adhesive is used, and differences in moisture permeability, heat expansion and the like occur in the drying process leading to a problem of a defect rate increase. As an alternative to solve the problems described above, methods of using non-aqueous adhesives instead of aqueous adhesives have been proposed.
Non-aqueous adhesives for a polarizing plate that have been proposed so far may be divided into radical-curable adhesives and cation-curable adhesives depending on the types of curing. Cation-curable adhesives have an advantage in that excellent adhesive strength is obtained for films made of various materials, but have many disadvantages in the manufacturing process due to a low curing speed and a low degree of curing. Radical-curable adhesives having acrylamide-based compounds as a main component have been proposed in order to solve the problems of such cation-curable adhesives. However, although radical-curable adhesives having acryl amide-based compounds as a main component have a higher curing speed than cation-curable adhesives, they have problems in that curing speeds decrease, and adhesive strength is reduced under high humidity conditions. Meanwhile, a process of preparing a polarizing plate has high moisture content since the process includes a wet process in which swelling, dyeing, orientation and the like of polyvinyl alcohol films are carried out in an aqueous solution, and therefore, in order to use the acryl amide-based adhesive for a polarizing plate, the polarizer needs to be hot-air dried or go through additional processes such as surface treatment such as plasma before applying the adhesive.
Accordingly, the development of radical-curable adhesives in which a curing speed and adhesive strength are not reduced even in high humidity environments has been required so that the radical-curable adhesives may be used in a polarizing plate without a separate treatment.